As a semiconductor rectifier which rectifies an input current to output the rectified current, a PiN diode having a p-n junction and a Schottky barrier diode (SBD) having a carrier potential barrier obtained by a difference between work functions of a semiconductor layer and a metal are known. Also, a Junction Barrier Schottky barrier diode (JBS) in which an impurity region (for example, p-type) having a conductivity type different from that of a semiconductor layer (for example, n-type) is arranged on the semiconductor layer surface to moderate an electric field applied to the semiconductor layer/metal interface in a Schottky barrier diode is known. Furthermore, a Merged PiN-diode Schottky-diode (MPS) in which a contact between an impurity region (for example, p-type) and a metal in a JBS is made an ohmic contact or close to an ohmic contact so as to cause minority carrier injection when a voltage exceeding a built-in potential (Vbi) between the impurity region and the semiconductor layer is applied, thereby obtaining a decrease of a resistance due to the conductivity modulation is known.
On the other hand, a wide bandgap semiconductor typified by, for example, silicon carbide (to be also be referred to as SiC hereinafter) is expected as a power semiconductor device material of next generation. The wide bandgap semiconductor has a band gap wider than that of Si, and has a breakdown electric field strength and a heat conductivity which are higher than those of Si. By utilizing the characteristics, a power semiconductor device with low loss and capable of operating at a high temperature can be realized.
In the MPS, by lowering a voltage which causes conductivity modulation, a reduction of resistance of forward bias characteristics can be realized and a large amount of current can be discharged by a low forward voltage when a forward surge current flows into the MPS. When a current larger than that in a stationary state flows into the MPS, the current causes crystal breakdown or junction breakdown of an electrode or the like by heat generation based on the energy equation: current×voltage=energy. However, if a large current can flow with a low forward voltage, heat generation energy can be suppressed to make it possible to decrease a breakdown probability.
US Patent Application Publication 2007/0278609 discloses an MPS using silicon carbide which lowers a voltage causing conductivity modulation by a combination of impurity regions having different widths.